Salts of aminopolycarboxylic acids such as N-2-carboxyethyl-aspartic acid (aminopolycarboxylates) have high chelating activity sequestering metal ions and may find application as chelating agents for use in the removal of toxic metals, concentrating and recovery of valuable metals, and fractional purification of metals.
However, when handled in solid states such as powders and granules, aminopolycarboxylates absorb atmospheric moisture to form viscous masses, with the result that for use as metal chelating agents or the like, there is room for contrivance avoiding such troubles to occur. Furthermore, regarding aminopolycarboxylates of low purity, these tend to separate out in aqueous solution necessitating to hold product concentrations sufficiently low. Thus, if aminopolycarboxylates could be handled stably, these compounds should become applicable to industrial uses such as detergent builders, soap additives, textile dyeing agents, plating chemicals, auxiliary bleaches, and photographic chemicals (photographic chelating agents) for the formulation of photographic processing compositions; therefore, there is room for technological sophistication in the art of stabilizing aminopolycarboxylates.
Meanwhile, Japanese Kokai Publication Hei-08-169866 discloses a technology of handling an iminocarboxylate salt in the form of an aqueous solution controlled to the isomer ratio of the aspartic acid skeleton of said iminocarboxylate salt to D-form/L-form (molar ratio)=1/0 to 0.7/0.3 or D-form/L-form (molar ratio)=0/1 to 0.3/0.7, the concentration of said iminocarboxylate salt being controlled to 40 to 70 weight % and the pH of the solution being controlled to pH 7 to 12. Japanese Patent Publication No. 2644977 discloses an iminocarboxylate salt aqueous solution controlled to the isomer ratio of the aspartic acid skeleton to D-form/L-form (molar ratio)=1/0 to 0.7/0.3 or D-form/L-form (molar ratio)=0/1 to 0.3/0.7, the concentration of said iminocarboxylate salt being controlled to 40 to 70 weight %.
However, whereas the iminocarboxylate salt has a structure containing 2 or 3 asymmetric carbon atoms, the aminopolycarboxylate has a structure containing one asymmetric carbon atom; in this respect, these compounds are different in chemical structure. Therefore, there is room for preventing the aminopolycarboxylate from caking and stabilizing the same.
Japanese Kokai Publication Hei-10-87580 discloses a monoaminocarboxylate with a total amount of nitrile compound and amide compound being controlled to 2% or less based on the monoaminocarboxylate. Japanese Kokai Publication Hei-10-87582 discloses a monoaminocarboxylate in which, based on the monoaminocarboxylate, the total amount of cyanide ion, hydrogen cyanide and its salt is controlled to not more than 100 ppm, the total amount of ammonium ion, ammonia and its salt controlled to not more than 1%, and the amount of formaldehyde controlled to not more than 1%.
However, these monoaminocarboxylates have a structure containing one or two asymmetric carbon atoms and are structurally distinct from aminopolycarboxylates. Moreover, as the technology for producing such monoaminocarboxylates, there is disclosed a process which comprises addition of cyanic acid and formaldehyde to the starting material amino acid and subsequent hydrolysis of the resulting addition compound under alkaline conditions, but the monoaminocarboxylate compound obtainable by such a production process has a pH value of about 13.5 to 14 and is not considered to have been sufficiently stabilized. Therefore, there is room for further technological sophistication in order that aminopolycarboxylates may be sufficiently stabilized to facilitate handling, insure long-term maintenance of product quality and, hence, may be adapted to various industrial applications.